Complementary metal-oxide-semiconductor (CMOS) technology has conventionally utilized n-type and p-type polysilicon as the gate electrode material. However, doped polysilicon may not be an ideal gate electrode material for advanced technology node applications. For example, although doped polysilicon is conductive, there may still be a surface region which can be depleted of carriers under bias. This depleted region may appear as an extra gate insulator thickness, commonly referred to as gate depletion, and may contribute to the equivalent oxide thickness. While the gate depletion region may be thin, on the order of a few Angstroms, it may become significant as the gate oxide thicknesses are reduced in advance technology node applications. As a further example, polysilicon does not exhibit an ideal effective work function (eWF) for both NMOS and PMOS devices. To overcome the non-ideal effective work function of doped polysilicon, a threshold voltage adjustment implantation may be utilized. However, as device geometries reduce in advanced technology node applications, the threshold voltage adjustment implantation processes may become increasingly complex.
To overcome the problems associated with doped polysilicon gate electrodes, the non-ideal doped polysilicon gate material may be replaced with alternative materials, such as, for example, metals, metal nitrides and metal carbides. For example, the properties of a metal, a metal nitride, or a metal carbide may be utilized to provide a more ideal effective work function for both NMOS and PMOS device, wherein the effective work function of the gate electrode, i.e., the energy need to extract an electron, may be compatible with the barrier height of the semiconductor material. For example, in the case of PMOS devices, the effective work function is approximately 5.0-5.2 eV, and in the case of the NMOS devices, the effective work function is approximately 4.1-4.3 eV. Accordingly, methods are desired for forming gate electrode for both NMOS and PMOS devices with preferred effective work functions.